1. Field of the Invention
The present invention relates to antennas. More specifically, the present invention relates to systems and methods for effecting delay in large phased array antennas.
2. Description of the Related Art
Phased array antennas are well known in the art. Phased array antennas are implemented with a large number of radiating elements, which are individually excited. Beam steering is effected by delaying the excitation of selected radiating elements. For most applications, smaller antennas are used. For these antennas, adequate delays can be supplied by adjusting the phase of the excitation signals supplied to the radiating elements, hence the term xe2x80x98Phased Array Antennasxe2x80x99 is used.
However, for spacecraft and other applications, large phased array antennas are required to obtain higher gain at longer detection ranges. In addition, various pulse compression techniques are typically used to achieve higher average radiated power for longer detection range while having range resolution for target discrimination or mapping resolution. Pulse compression results in signals with wider bandwidths. When the size of the phase array antenna becomes relatively large and wide bandwidth waveforms for high range resolution are needed, some form of true time delay (TTD) antenna beam steering, as opposed to phase only beam steering is needed to accommodate the size, bandwidth and scan angle requirements of the antenna.
Conventionally, TTD beam steering has been achieved by using delay lines to feed the radiating elements. Coax, waveguide and fiber optic cables have been used to provide the necessary TTD. However, coaxial delay elements are heavy and induce dispersion in the output beam. Dispersion is to due a failure to transmit each frequency component at a uniform speed and leads to a corruption of the wavefront.
The use of waveguides and fiber optic cables could be found to be expensive and impractical (e.g., too heavy) for space and other applications.
When the RF signal has a wide instantaneous bandwidth, then typically physical delays are needed to point an antenna array. For the case where wide bandwidth is obtained using a time varying frequency waveform, then techniques that do not use physical delays in the signal path can be utilized.
Hence, a need remains in the art for a system and method for delaying excitation of radiating elements in large antennas with respect to wide bandwidth time varying frequency signals and wide scan angles without using physical delay elements.
The need in the art is addressed by the beam steering system and method of the present invention. The inventive system is adapted for use with an antenna having an array of radiating elements and includes a plurality of signal sources. Each source is adapted to provide a signal having a predetermined frequency offset signal for an associated radiating element or set of radiating elements. A plurality of first mixers is provided. Each of the first mixers is coupled to receive an excitation signal as a first input and the output from a respective one of the sources as a second input. The output of each mixer is coupled to a respective subset of the antenna radiating elements.
In effect, each of the sources provides a virtual time delay. In a specific embodiment, the virtual time delay source includes a direct digital synthesizer that has a digital-to-analog (D/A) converter included in said synthesizer to receive the output thereof. A second mixer is included in each source for mixing the output of the converter with the output of a supplemental signal source. The output of the second mixer is supplied to a respective one of the first mixers.
The invention provides a technique for generation of a virtual time delay (VTD) for phased array antenna beam steering without sending the waveform through an actual (physical) time delay. The technique adjusts the phase and frequency of an RF waveform that is spread out in frequency and time such that each frequency arrives at the radiating elements of a phased array antenna with the phase that will steer the antenna beam in the desired direction. The relative signal phase at each radiating element is made to be independent of the signal carrier frequency and thus the antenna beam pointing is made independent of RF carrier frequency.